Digital clock

DIGITAL CLOCK

Overview

This is a bare-metal C project based on the Microchip PIC18F4550 microcontroller that implements a digital clock. The system uses multiplexed seven-segment displays to show the current time and provides a push-button interface for user configuration. The configured time is stored in the internal EEPROM memory.

Features

• Modular driver-based firmware architecture
• Timer2-driven system tick for timekeeping and scheduling
• Multiplexed seven-segment display control
• Push-button interface for user configuration
• EEPROM-based non-volatile time storage

Hardware

• Microchip PIC18F4550 microcontroller with a 16 MHz external crystal oscillator
• Multiplexed common-cathode seven-segment display
• NPN transistors for digit control
• Push-button interface with pull-up resistors

Software architecture

The firmware is organized using a modular architecture that separates hardware drivers from application logic.

The modules include:

• GPIO HAL for low-level pin control
• Display driver for multiplexed seven-segment control
• Button driver for user input handling
• System tick dirver providing a 1 ms time base
• EEPROM driver for non-volatile data storage
• Clock driver for real-time management
• UART driver for testing and debugging

How It Works?

The system operates on a 1 ms system tick generated by the Timer2 peripherial. The tick drives a cooperative scheduler responsible for executing periodic tasks and maintaining the system time. The scheduler manages key tasks such as button input processing, display multiplexing, and timekeeping. This approach ensures accurate timing efficient CPU utilization.

Challenges & Design Decisions

Display ghosting

My initial design used common-anode displays driven by PNP transistors through a Darlington array, chosen to limit the current going through the PIC's GPIO pins. This setup caused visible ghosting: the Darlington array's slow switching times and high saturation voltage drop meant that the previous digit didn't fully turn off before the next one turned on during multiplexing.

I redesigned the digit-drive stage with common-cathode displays driven directly by NPN transistors. The faster switching produced cleaner digit transitions and eliminated the ghosting entirely. This taught me that in embedded development, firmware correctness alone isn't enough — the underlying circuit topology can make or break a working system.

Build Instruction

make all
make flash
make clean

Schematic